Marsh Foraminifera and Arcellaceans of the Lower Mississippi Delta: Controls on Spatial Distributions

Abstract

Foraminifera and arcellaceans (thecamoebians) were examined from 73 surface samples collected to represent four vegetation zones (I-IV) that have been defined in the Mississippi Delta Plain. Previous studies of benthonic foraminifera had not differentiated these environments. Saline marshes (I) are characterized by a vertical zonation typical of most marshes, with a variety of estuarine species in the low and trochamminids in the upper marsh. Brackish marshes (II) are dominated by a completely different set of species, all agglutinated, but the vertical zonation is weak, probably because of the low vertical gradient. Emergent freshwater or intermediate marshes (III) are dominated by arcellaceans but there are significant numbers of foraminifera, indicating episodic marine incursions. The freshwater floating zone (IV) has only arcellaceans. A previous study of arcellaceans living in the lacustrine environment below the floating reveals a different arcellacean fauna than that observed above it. These data suggest that foraminifera have potential application as sea-level indicators in the delta region and that fossil remains of these protozoans can be used to differentiate coastal vegetation zones in the fossil record. INTRODUCTION The lower Mississippi delta region is one of the most intensively studied Quaternary sections in the world and benthonic foraminifera have been used frequently by various investigators. Surprisingly, although some overall distribution patterns are known, detailed relations between wetland benthonic foraminiferal assemblages and vegetation assemblages within and between regions is not well known. All previous workers have listed a group of species referred to as marsh species but have not tried to subdivide them. Also, with the exception of Haman (1982, 1990), arcellaceans (or thecamoebians) have been virtually ignored in investigations of freshwater areas of the lower delta. We have carefully sampled most of the major vegetation zones and, in marine marshes, we were careful to sample the entire vertical range of the salt marshes to detect vertical zonations that have been observed elsewhere (Scott and Medioli 1980a). Previous cores from the delta, examined several years ago (unpublished student data from Dalhousie University and Collins 1988), indicated the presence of discrete assemblages of species. However, without data on their modem distribution little interpretation of the core assemblages was possible; this is what prompted the present study. Using data from other areas, it was possible to speculate on assemblage characteristics (i.e. elevation, salinity requirements); however, because the Mississippi delta region is much more complex than most regions with the low elevation gradient and the many vegetation types, it was not possible to have confidence in our interpretations. PHYSIOGRAPHY Physiographically, this system is a low lying coastal plain with an extremely small vertical gradient, which has promoted the development of an extensive wetland system ranging from saline to freshwater. In this study, we have attempted to sample each of the wetlands except the freshwater cypress swamps. Salinities and temperatures in the wetlands are strongly influenced by atmospheric conditions and one time measurements of these parameters are not so useful; however salinity values were obtained where possible during the sampling. The tidal range is low (30cm) but storm surges generated by hurricanes can raise the sea level from .5m to over 7m (Penland et al. 1988). These authors also state that the average wind-generated waves are low-energy, such that the coastline is considered a storm-dominated system.